In recent years, there have been extended optical communication networks capable of high-speed and large-capacity data communication. Such optical communication networks are expected to be mounted on commercial apparatuses, in the future. Therefore, there has been a need for optical data transmission cables (optical cables) for electrical inputting and outputting which are usable in the same ways as those of existing electric cables, as applications for realizing high-speed and large-capacity data transmission, countermeasures against noises and data transmission among boards in apparatuses. It is desirable that film optical waveguides are employed as such optical cables, in consideration of flexibility.
An optical waveguide is constituted by a core with a higher refractive index and a cladding with a lower refractive index which is provided around the core in contact with the core and is adapted to propagate optical signals incident to the core while causing total internal reflection thereof at the interface between the core and the cladding. Further, a film optical waveguide has flexibility, since the core and the cladding therein are made of flexible polymeric materials.
In cases where such a film optical waveguide with flexibility is employed as an optical cable, it is necessary that the film optical waveguide is aligned with and optically coupled to a photoelectric conversion device (an optical element). Such an optical element is for converting electric signals into optical signals and emitting them or for receiving optical signals and converting them into electric signals, and such an optical element can be a light emitting device in the light inputting side or a light receiving device in the light outputting side. The alignment thereof should be performed with high accuracy, since it affects the optical coupling efficiency.
FIG. 21 illustrates an exemplary structure where a film optical waveguide and an optical element are aligned with and optically coupled to each other in an optical module.
The optical module 100 illustrated in FIG. 21 is constituted to include an optical waveguide 101, an optical element 102 and a support board 103, at its optical-incidence side or optical-emission side end portion. The optical waveguide 101 is secured near its end portion to the support board 103 through adhesion or the like, in a state where the positional relationship between the end portion of the optical waveguide 101 and the optical element 102 is fixed.
The support board 103 has a step which causes its surface on which the optical element 102 is mounted and its surface to which the optical waveguide 101 is secured (adhered) to be different surfaces. Further, the end surface of the optical waveguide 101 is not perpendicular to the optical axis (the center axis along the longitudinal direction of the core) and is obliquely cut to form an optical-path changeover mirror. Accordingly, signal light which has been transmitted through the core portion of the optical waveguide 101 is reflected by the optical-path changeover mirror to change the direction of its proceeding and, then, is emitted toward the optical element 102.
In the aforementioned structure in FIG. 21, a gap is induced between the lower surface of the optical waveguide 101 and the upper surface of the optical element 102. Further, on the side of light emission from the optical waveguide 101, as illustrated in FIG. 22, the emitted light which is emitted from the end portion of the optical waveguide 101 toward the light receiving device 102 becomes diffused light, not parallel light. Accordingly, if there is a gap between the lower surface of the optical waveguide 101 and the upper surface of the optical element 102, this will induce light deviated from the light receiving surface of the light receiving device 102, thereby resulting in an optical loss. Further, although not illustrated, on the side of light incidence to the optical waveguide 101, the incident light from the light emitting device 102 is diffused, which increases the quantity of light which can not be coupled to the core portion of the optical waveguide 101, thereby resulting in an optical loss.
As methods for suppressing such optical losses, there are a method which positions the optical element and the optical waveguide such that the distance therebetween is made smaller, a method for condensing light with an optical component such as a lens or a prism, and the like. However, the former method has a large influence caused by the variation of mounting and the difficulty in securing a space for bonding wiring for the optical element. Further, the latter method has an increase in the number of components.
Further, Patent Documents 1 and 2 disclose structures for embedding a resin with a high refractive index in the gap between a light emitting device and an optical waveguide for adhering and securing the optical waveguide with the resin. This structure can suppress the undesirable reflection at the interface with the aforementioned resin layer, thereby increasing the optical coupling efficiency. Further, the structure for embedding a resin with a higher refractive index in the gap between the optical element and the optical waveguide can also reduce the light diffusion between the light emitting device and the optical waveguide, and this effect is expected to increase the optical coupling efficiency.    Patent Document 1: Japanese Patent Unexamined Publication “JP-A No. 2000-214351 (date of publication: Aug. 4, 2000)”    Patent Document 2: Japanese Patent Unexamined Publication “JP-A No. 2000-9968 (date of publication: Jan. 14, 2000)”